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- Кто это?

- Это мой брат.

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The question “Кто это?” is very wide and doesn’t require any special information about the person. So, “Кто это? – Это мой брат (Who is that? – That is my brother)” is a possible conversation. The question “Кто он?” doesn’t work for “Это мой брат” because this question normally requires  information about person’s profession or occupation. The variant “Антон” doesn’t work to complete the question because when we use names after “кто” we add a “blank” word “такой/ такая/ такие”: “Кто такой Антон?”

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- Кто вы по профессии?

- Я аналитик.

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The idea of this question is ‘who are you?’. Literally, we don’t ask “What is your profession?” in Russian. Instead we say “Who are you? (Кто вы?)” and then add the explanation “according to your profession (по профессии)“. Just the question “Кто вы?” very often implies information about person’s occupation and in many situations is equal to the question “Кто вы по профессии?”

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2 answers from the list are possile:

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- Кто он?

- Он аналитик. / – Он студент.

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Normally the question “Кто вы?” implies information about person’s profession or occupation and in many situations is equal to the question “Кто вы по профессии?” So, possible answers are “аналитик” (analyst), “студент” (student). The variants “Антон”, “мой брат” (my brother) could be answers for the question “Кто это?”. This question is very wide and doesn’t require any special information about a person.

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